1. Field of the Invention
This invention relates generally to an apparatus and methods for reducing vibrations. More particularly, this invention relates to an active-passive hybrid constrained layer.
2. Background of the Invention
Controlling structural vibration is important in numerous technical fields. Examples of such fields include, without limitation, aerospace, automotive, machinery, and numerous other specific applications where vibration control is desirable. A background discussion of active controls and passive controls is found in U.S. Pat. No. 5,838,092, hereby incorporated by reference in its entirety.
Active constrained layer (ACL) damping treatments have been studied by various researchers. See Agnes, G. and Napolitano, K., 1993, xe2x80x9cActive Constrained Layer Viscoelastic Damping,xe2x80x9d Proceedings 34th SDM Conference, pp. 3499-3506; Baz, A., 1993, xe2x80x9cActive Constrained Layer Damping,xe2x80x9d Proceedings of Damping 93, San Francisco, Calif., Vol.3, pp. IBB 1-23; Shen, I. Y., 1993, xe2x80x9cIntelligent Constrained Layer: An Innovative Approach,xe2x80x9d Intelligent Structures, Materials, and Vibrations, ASME DE-Vol. 58, pp. 75-82; Huang, S. C., Inman, D. J. and Austin, E. M., 1996, xe2x80x9cSome Design Considerations for Active and Passive Constrained Layer Damping Treatmentsxe2x80x9d, Smart Materials and Structures, Vol. 5, pp. 301-313; Liao, W. H. and Wang, K. W., 1997, xe2x80x9cOn the Analysis of Viscoelastic Materials for Active Constrained Layer Damping Treatments,xe2x80x9d Journal of Sound and Vibration, Vol. 207, pp. 319-334; and Liao, W. H. and Wang, K. W., 1997, xe2x80x9cOn the Active-Passive Hybrid Control Actions of Active Constrained Layers,xe2x80x9d ASME Journal of Vibration and Acoustics, Vol. 119, pp. 563-572.
Such systems generally consist of a piece of viscoelastic material (VEM) sandwiched between an active piezoelectric layer and a host structure. The main purpose of using a piezoelectric coversheet is that its active action enhances the viscoelastic layer damping ability by increasing the VEM shear angle during operation (the so-called enhanced passive damping action). When the active action fails, significant passive damping could still exist in ACL (it becomes a passive constrained layer (PCL) configuration), which would be important for fail-safe reasons. On the other hand, because the VEM will reduce the active authority of the piezoelectric layer, it would be more effective to use the Enhanced Active Constrained Layer (EACL) concept disclosed in Liao, W. H. and Wang, K. W., 1996, xe2x80x9cA New Active Constrained Layer Configuration with Enhanced Boundary Actions,xe2x80x9d Smart Materials and Structures, Vol. 5, pp. 638-648; Liao, W. H. and Wang, K. W., 1998, xe2x80x9cCharacteristics of Enhanced Active Constrained Layer Damping Treatments with Edge Elements, Part 2: System Analysis,xe2x80x9d ASME Journal of Vibration and Acoustics, Vol. 120, pp. 894-900; and Liu, Y., and Wang, K. W., 1999a, xe2x80x9cA Non-dimensional Parametric Study of Enhanced Active Constrained Layer Damping Treatments,xe2x80x9d Journal of Sound and Vibration, Vol. 223, No.4, pp. 611-644; or the separate active and passive designs of (Lam, M. J., Inman, D. J. and Saunders, W. R., 1998, xe2x80x9cVariations of Hybrid Damping,xe2x80x9d Proceedings of SPIE on Smart Structures and Materials Vol. 3327, pp. 32-43, if high active action is needed. Therefore, it has been recognized that the applications best suitable for ACL treatments are those that can utilize significant damping from the VEM, rather than from direct piezoelectric-structure interactions.
Given the above observations, it would be important and desirable to optimize the open-loop characteristics (the so-called baseline structure without active action) of the ACL treatment, as well as the system""s closed-loop behavior from the enhanced passive damping action. In both cases, the constraining layer material property plays an important role. An ideal constraining layer for ACL would be a material with high stiffniess, lightweight, and high active authority. So far, the constraining layer in ACLs has been limited to piezoelectric materials (e.g., PZT ceramics or PVDF polymer) because of their active features. PZT materials are in general much better than PVDF polymers for this purpose. Nevertheless, having a density similar to steel (relatively heavy) and a modulus close to aluminum (moderate stiffness), PZTs are not ideal as constraining materials (Lam, M. J., Inman, D. J. and Saunders, W. R., 1998, xe2x80x9cVariations of Hybrid Damping,xe2x80x9d Proceedings of SPIE on Smart Structures and Materials Vol. 3327, pp. 32-43). Due to this limitation in the original baseline structure, the open-loop damping ability of an ACL system, in general, is less than that of an optimally designed PCL system.
As can be seen from the foregoing discussion, problems remain with active constraining layers. Thus a need in the art exists for a constraining layer that improves upon the prior art.
It is therefore an object of the present invention to provide a method and apparatus for a constrained layer damping treatment that improves over the state of the art.
A further object of the present invention to provide a method and apparatus for an improved constrained layer that optimizes the open-loop characteristics of an ACL treatment.
A further object of the present invention to provide a constrained layer that improves the systems closed-loop behavior from an active-passive damping action.
A further object of the present invention to provide a constrained layer that is capable of using piezoelectric materials.
A still further object of the invention is to provide a constrained layer capable of meeting damping requirements while requiring less added weight.
A still further object of the invention is to provide a constrained layer capable of use with different host structure strain distributions.
Other objects of the invention will become apparent from the description of the invention in that which follows.
The invention is a novel method and apparatus for an active-passive hybrid constraining layer (HCL) surface damping treatment. The invention improves the damping performance of traditional active constrained layer systems. Instead of using a pure piezoelectric constraining layer or other active layer, passive and active materials are used together to constrain the viscoelastic material layer.
One aspect of the invention provides for selecting a passive constraining material and assigning appropriate lengths for the active and passive constraining parts in order to optimize the damping effect of the constraining layer.
Another aspect of the invention provides for separating the passive constraining material and/or the active constraining material into one or more sections in creating the constrained layer in order to improve the damping effect.